ABSTRACT The two subtypes of estrogen receptor, ERa and ERb, carry out non-overlapping functions in reproductive biology. ERb is capable of both interfering with ERa function and exerting its biological activity independent of ERa. It remains unclear how the ERa-independent and ERa-competing functions of ERb are regulated. We recently made the pioneering discovery of an ERb-specific phosphotyrosine switch that regulates ERb function in reproductive organs. Based on our preliminary work, we hypothesize that a single phosphotyrosine switch toggles between two different ERb functions: while phosphorylated ERb is active in its ERa-independent function in ovarian folliculogenesis, unphosphorylated ERb is more potent in its ERa- competing function in mammary gland development. With a repertoire of unique tools including a phospho- specific antibody and a knockin animal model, we are ideally positioned to test this novel hypothesis by investigating how the phosphotyrosine switch differentially regulates two distinct ERb activities. The concept of a molecular toggle between two distinct activities of ERb represents a previously unappreciated mechanism that integrates control of two subtype-specific ERb activities through a single signaling switch. In addition, the unique technical tools and experimental models developed in our work will greatly promote in-depth studies of endocrine functions in reproductive biology.